19. Radiation Heat Transfer (Heat transfer by thermal radiation)

All bodies radiate energy in the form of photons moving in a random
direction, with random phase and frequency. When radiated photons
reach another surface, they may either be absorbed, reflected or
transmitted. The behavior of a surface with radiation incident upon
it can be described by the following quantities:

Reflective energy may be either diffuse or specular (mirror-like).
Diffuse reflections are independent of the incident radiation angle.
For specular reflections, the reflection angle equals the angle of
incidence.
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